1. Field of Invention
The present invention relates to an exhaust gas control actuator suitable for feed-back controlling the feed of secondary air fed to an exhaust piping by detecting the concentration of exhaust gas from an engine.
2. Prior Art
In general, it has been known that, in order to simultaneously decrease HC, CO and NO.sub.x in the gas exhausted from an engine, the air-fuel ratio of the exhaust gas flowing into a catalytic device, particularly, a three-way catalytic converter, should be maintained within a narrow range centered around the theoretical or stoichiometrical air-fuel ratio. Consequently, an air-fuel ratio of a carbureter of an engine is set on the side richer than the theoretical air-fuel ratio, a supply of secondary air is blown into an exhaust manifold of an exhaust piping from an air pump, led to a three-way catalytic converter via an exhaust pipe disposed downstream, and a feed-back is made to control the feed of secondary air so that an air-fuel ratio of an exhaust gas entering the three-way catalytic converter can be maintained within a narrow range centered around the theoretical air-fuel ratio by a signal from an exygen concentration detector (hereinafter referred to as an "O.sub.2 sensor") provided at an inlet of the three-way catalytic converter, thereby enabling to simultaneously decrease HC, CO and NO.sub.x in the exhaust gas and increase the power performance of the vehicle.
However, with the conventional system for supplying secondary air to an exhaust piping, such problems are presented that it is difficult to secure the durability of the catalyst due to the influence of the output characteristics of an O.sub.2 sensor as will be described below.
Namely, although an exhaust gas in the exhaust pipe whose apparent air-fuel ratio is equalized to the theoretical air-fuel ratio by being fed with secondary air and an exhaust gas produced from theoretical air-fuel mixture when a mixture having the theoretical air-fuel ratio is burned in an engine have the air-fuel ratio identical with each other, they differ in composition from each other. The former is higher in values of H.sub.2, CO and O.sub.2 than the latter. Furthermore, an O.sub.2 sensor utilizing zirconia (ZrO.sub.2) is affected not only by the concentration of O.sub.2 but also concentrations of H.sub.2, CO, particularly, H.sub.2, and the air-fuel ratio at which the output voltage has sharply risen is shifted to the side leaner than the theoretical air-fuel ratio. Therefore, the O.sub.2 sensor shows such an output characteristics that the signal change point of the O.sub.2 sensor disposed in an exhaust piping fed with secondary air is shifted to the side leaner than the theoretical air-fuel ratio.
Additionally, a catalyst has O.sub.2 storage action to a certain extent, and hence, even if the air-fuel ratio of the exhaust gas varies by about .+-.0.5 with respect to the theoretical air-fuel ratio, HC, CO and NO.sub.x can be simultaneously purified at a high purification rate as in the case of the exhaust gas having the theoretical air-fuel ratio. However, when the concentrations of H.sub.2, CO and HC are high in the composition of the exhaust gas, the average working temperature of the catalyst becomes higher, which, therefore, leads to a problem of the overheat of the catalyst under a high load operation for a long period of time, thus requiring consideration for protecting a pellet carrying the catalyst, a container and the like against overheating.
Further, the three-way catalytic converter is durable in the use conditions such as always subjected to a reducing environment (the rich exhaust gas) or an oxidizing environment (the lean exhaust gas) alternately and the deterioration of the purification performance thereof is low. However, if the rate at which the three-way catalytic converter is subjected to the oxidizing environment is high, then the durability thereof decreases, the deterioration of the purification performance thereof is sped up, and, in general, the durability decreases to scores of thousands kilometers calculated in terms of the running distance. As described above, the fact that the signal change point of the O.sub.2 sensor is shifted to the side leaner than the theoretical air-fuel ratio increases the rate at which the three-way catalytic converter is subjected to the oxidizing environment which, further, makes the securing of the durability of the catalyst more difficult in cooperation with the fact that, owing to the heat generation by the oxidation, the working temperature of the catalyst becomes higher than the case where the exhaust gas is treated under the true theoretical air-fuel ratio.
Additionally, the speed of response to the O.sub.2 sensor installed in the exhaust piping, in the case the air-fuel ratio becomes higher than the theoretical air-fuel ratio and a rich signal of the fuel shifts to a lean signal of the fuel, shows a moderately falling characteristics as indicated by a characteristic curve A in FIG. 1 wherein the time T is given as an abscissa and the output voltage V of the O.sub.2 sensor is given as an ordinate, and in the case contrary to the above, shows a sharply rising characteristics as indicated by a characteristic curve B in FIG. 1. Namely, with regard to the time lag characteristics of the O.sub.2 sensor, the time lag is different in magnitude depending upon the direction of the change-over of the output signal. In order to precisely control the air-fuel ratio of the exhaust gas entering the catalyst as centered around the theoretical air-fuel ratio, it is desirable to set a controlling amplitude centered around the theoretical air-fuel ratio within a smallest possible range on the plus and minus sides.
Furthermore, in the secondary air control system as above, the following requirement has been made from the aspect of the operation of the engine. Namely, in order to meet the demand in the output without increasing the volume of the engine as compared with the size of the vehicle, render the vehicle light in weight and improve the fuel consumption, it is desirable to make the air-fuel ratio of the carbureter rich up to about 12.5-11.5 for example at the time of the acceleration or the high output condition of the vehicle. Additionally, when an air pump whose delivery is directly proportional to RPM of the engine is used in respect of the secondary air supply, it is desirable that the feed rate of secondary air can be controlled in accordance with the change in air intake amount of the engine even under the constant RPM of the engine, and further, that the feed of secondary air is as quickly as possible and appropriately controlled through feedback to purify the exhaust gas under the various conditions of the engine such as the cold starting with insufficient warming up. Further, it is also desirable that the supply of secondary air is interrupted for the protection of the catalyst when the catalyst is overheated due to the operation under high load for a long period of time, and, that the supply of secondary air is performed at its maximum degrees when there is a possibility of the catalyst overheat at the engine braking after the high speed running, thereby simply effecting the protection of the catalyst.